I think you are confusing the terms. The TBF is the group of PDCHs allocated to one user.
The PDCH is one GPRS timeslot.
The throughput of the PDCH depends on the coding schemes. If 2 users are sharing one PDCH, with user 1 using it 50% of the time at 30kbps and the other user using it 50% of the time at 40kbps, then the PDCH throughput is 35kbps.
One user can have for example 4 PDCH at 50kbps each.? In this case, the TBF throughput is 200kbps.
Hope it clarifies the concept.